In burners, NO.sub.x emissions rise exponentially with combustion temperature. These emissions typically are reduced by lowering combustion temperatures. In some cases this is accomplished by combusting the fuel with an increased amount of excess air (fuel-lean mixture), with the overall amount of combustion air substantially higher than the stoichiometric ratio. In other cases where low excess air is important for the efficiency of the operation, the emissions are reduced by fuel-staged combustion, with high excess air at the first stage and secondary fuel burning and consuming excess air downstream of the first stage.
One example of a system using excess air to reduce NO.sub.x emissions is disclosed in the article "The Development of a Natural Gas-Fired Combustor for Direct-Air" from the 1992 International Gas Research Conference. In this burner system, the fuel and gas are premixed and then injected in the combustion chamber. The air-fuel mixture is adjusted to provide whatever amount of excess air is desired to lower the temperature so that NO.sub.x emissions are minimized. One of the drawbacks of this system, however, is low turn-down and the danger of explosions upstream from the combustion chamber, for example in the burner.
In U.S. Pat. No. 5,102,329, a low NO.sub.x burner is disclosed, in which mixing of fuel gas and combustion air to the extent necessary for combustion in the burner is precluded. In this burner, fuel tubes or spuds are arranged over slots in a burner plate to discharge fuel gas therethrough at high velocities. Combustion air also is discharged from the burner through these slots. Although some mixing of fuel gas and combustion air (controlled exclusively by fuel gas jet entrainment of the combustion air) occurs along the boundary line between each cone-shaped fuel gas jet and the air, the space volume where this mixing occurs is negligible. In addition, the flow pattern in this area has a velocity component in the downstream direction that many times exceeds the propagation velocity of the flame. Accordingly, any flame flashback from the combustion chamber is mostly precluded and, if it occurs at extremely low loads, does not represent a danger for the burner operation.
Although the above systems advantageously reduce NO.sub.x emissions and, in the latter case, minimize the possibility of flame flashback, they are under certain conditions subject to combustion driven pulsation, which should be avoided. In burners generally, the combustion pulsations typically occur at a frequency of about 0.5-200 Hz due to the particular characteristics of the turbulence in the air supply, or numerous resonance modes of the system. It has been found that when heat of combustion is applied rapidly and uniformly to the mixture of fuel and air downstream of the burner in the area of combustion, it creates favorable conditions for the flame front to oscillate toward and away from the burner at a frequency determined by the system. This leads to vibrations, and causes resonance of the hardware of the furnace. These vibrations and resonance problems are of particular concern in large combustion devices.
U.S. Pat. No. 5,460,512 addresses these problems by providing a burner construction in which local oscillations of flame front generated in the combustion chamber are at different frequencies which are not synchronized, so that vibrations are greatly dampened and resonance problems in the furnace minimized or eliminated. The burner includes a burner plate having a plurality of slots from which fuel gas jets and combustion air are discharged. The slots are arranged such that the width of the recirculation zones between adjacent slots substantially varies between the central region of the burner plate and its perimeter. With this construction, the local ignition patterns vary such that local oscillations of flame front occur at different frequencies so that vibrations are greatly dampened and resonance problems in the furnace minimized or eliminated. In applications where high excess air is not desirable, such as boiler applications, the burner is modified by providing a secondary fuel and flue gas injection assembly to form a two-stage burner. The secondary injection assembly includes a plurality of discrete fuel and flue gas injection tubes arranged around the primary air and fuel gas discharge assembly. The secondary fuel is directed radially inward and downstream from the burner plate. At first the secondary fuel entrains partially cooled products of combustion surrounding the flame and then mixes with the remaining combustion air and burns in a secondary combustion zone. The resulting delay in the combustion of the secondary fuel gas and the involvement of partially cooled combustion products again in the combustion lowers peak combustion temperature, which in turn reduces the NO.sub.x formation in the second or downstream combustion zone.
The design of this kind of low NO.sub.x burner is dependent on a number of parameters, including target NO.sub.x emission level, types of fuels fired, furnace size, burner geometry, and cost. A particular burner for a specific application has a limited range of parameter variability for optimization. One of the most important limitations is the maximum size of the combustion device. There are several aspects in the known designs that limit its application, especially when very high heat inputs (typically over 100 million Btu per hour from a single device) are required. The first is a relatively large size of the device that sometimes makes it difficult to fit the burner within the available space at the front of the boiler. Second, a larger burner also requires a substantially larger air plenum at the front of the furnace that encompasses the burner body to provide proper air distribution across the burner. These wind boxes take up valuable real estate at the boiler front, often at the expense of boiler service area. Third, the flame generated by the burner is overall axially symmetrical. This creates a problem if the furnace is rectangular with a high aspect ratio and a high heat release per unit of furnace cross-section. Another limitation of the known design is the difficulty in accommodating firing of more than one gaseous fuel and one liquid fuel, as there is only one convenient location in the center of the burner for the liquid fuel gun.